Originally Posted by Savington

I don't see any mention of turbine flow, which is arguably the more important factor when you are looking at detonation resistance. Compressor maps only tell you half the story.

Your guesses are quite good, BTW - 320-340whp on 93 octane is just about the real-world limit. A little lower on CA91.

Remember that the 6258 and 6758 use the same turbine side, so the only reason to select the 6758 would be for high-boost efficiency. In the real world, those two turbos are going to make extremely similar power figures up to around 20-22psi. Only past that, where the efficiency of the 6258 falls off dramatically, does the 6758 begin to actually make a difference.

If you only want 350whp, the 6258 is the correct option. There are multiple examples of that turbo making a legit, uncorrected 415whp+ if all the other ducks in your pond are in a row. If you must have 400-450whp, then the 6758 is recommended. The 6258 might not stretch to 450whp no matter what, but the 6758 will get there with its pants on its head.

RPM really doesn't make a big difference, it's all about airflow. People will be along shortly to disagree, I'm sure. IMO, the turbo doesn't care whether you make 400whp at 6000rpm or 8000rpm, it's the same quantity of air moving through the system either way. Yes, the VE is slightly different. Call me when you have a verified accurate VE table for a BP engine and I (and several others) will pay you a princely sum for the data. Until then, you're guessing, and the difference between 6k and 8k is within the noise of that guess.

7000rpm is the correct answer when the question is "what RPM limit do I want for long-term track reliability". There are specific reasons for revving higher, all of which involve a combination of "I hate money" and "I want to win the races I enter".